This application claims the priority of German patent document 196 45 646.0, the disclosure of which is expressly incorporated by reference herein.
The invention relates to a device for controlling the steering angle of a vehicle that has an operating element for adjusting steering angle as a function of a parameter, such as the deflection of the operating element or the force exerted on the operating element.
Conventional devices of this type use a steering wheel as the operating element. The steering angle is adjusted by direct mechanical transmission of the steering wheel rotary movement to the vehicle wheels. It is also known to provide a steering wheel (or another user-operable operating element) that is mechanically decoupled from the steering movement of the wheels; by actuation of such a steering wheel, a steering angle adjusting unit can be controlled which in turn performs the corresponding steering angle adjustment of the wheels. Such arrangements with manually operable operating elements are described in the article by H. Bubb, "Arbeitsplatz Fahrer--Eine ergonomische Studie" [The Driver Workstation: An Ergonomic Study] Automobil-Industrie 3/85, page 265, and in U.S. Pat. No. 3,022,850. In the latter patent in particular, it is proposed to amplify the deflection signal of a control lever used for adjusting the steering as a function of the vehicle speed, preferably inversely proportionally to the square of the lengthwise velocity. Thus, a given control lever deflection results in essentially constant transverse acceleration, independent of the lengthwise speed of the vehicle.
In the article by P. Branneby et al., "Improved Active and Passive Safety by Using Active Lateral Dynamic Control and an Unconventional Steering Unit," 13th International Technical Conference on Experimental Safety Vehicles, Proceedings Vol. 1, Nov. 4-7, 1991, page 224, a steering angle adjusting device is described in which a nonlinear steering angle characteristic is provided as a function of the deflection of the operating element, so that the setting of larger steering angle changes takes place with greater sensitivity than the setting of smaller steering angle changes. In other words, a given change in deflection of the operating element produces a smaller change in steering angle in the range of small steering angles than in the range of large steering angles. At the same time, the sensitivity of the steering angle adjustment is selected variably as a function of the vehicle speed. That is, it increases with vehicle lengthwise speed.
German patent document DE 44 03 379 A1 describes a steering system in which adjustment of the steering angle takes place with progressively increasing dependence on the steering wheel angle by actuating a conventional steering wheel. With increasing vehicle speed, a characteristic curve of the steering angle of the vehicle wheels that becomes flatter is chosen as a function of the steering wheel angle, so that the sensitivity of the steering decreases.
Unpublished German patent document DE 195 48 713 C1 describes a steering angle control device of the type mentioned above in which steering angle adjustment is performed as a function of the degree of actuation of the corresponding operating element, with a sensitivity that decreases with decreasing coefficient of friction and/or higher vehicle longitudinal speed.
One object of the present invention is to provide a steering angle control device of the type referred to previously, which permits steering of the vehicle that is safer from the driving dynamic standpoint and is ergonomically advantageous, especially with an operating element in the form of a control stick instead of a conventional steering wheel.
This and other objects and advantages are achieved according to the invention by performing the steering angle adjustment as a function of the degree of actuation of the operating element, with a steering ratio that depends upon the vehicle speed. The ratio is constant in a lower speed range while in a middle speed range it increases at least linearly, and in an upper speed range it increases at most linearly. The term "steering ratio" is the ratio of the rate of change of the actuating value to the rate of change of the steering angle (corresponding to the inverse of the sensitivity of the steering angle adjustment). Preferably the steering ratio in the medium speed range increases progressively more than linearly and in the upper speed range is decreasingly less than linearly increasing.
The constant steering angle ratio in the lower speed range means that at these low vehicle speeds the full steering angle of the vehicle wheels is available, for example for parking and maneuvering, as well as for utilizing the maximum physically possible transverse acceleration at the corresponding vehicle speed. The linear or more progressive pattern of the steering ratio in the medium speed range means that the maximum possible steering angle of the wheels, corresponding to the maximum value of the actuating parameter, decreases successively with increasing vehicle speed. In this manner, it is assured that the vehicle does not react too abruptly to a given actuation of the operating element, and as a result remains controllable in terms of driving dynamics. In the high speed range, the percentage of the slip angle at the steering angle increases continuously because of the increasing minimal curve radius that can be traveled. As a result, the steering angle that is required in the current situation for rounding a curve at the maximum transverse acceleration for an understeering vehicle with increasing longitudinal velocity approximately tends toward the value of the slip angle that permits development of a maximum lateral force on the wheels. It develops that with this behavior the steering ratio pattern, which according to the invention increases linearly or less degressively shows good correlation with higher vehicle speed.
In one embodiment of the invention, the steering ratio converges with increasing vehicle speed toward a maximum value whose corresponding maximum steering angle value corresponds to the slip angle value that permits maximum lateral force development. With these measures, the device is very well suited for the property mentioned above, namely that the steering angle for an understeering vehicle, with increasing speed, is intended to approach approximately the slip angle value that permits maximum lateral force, in order to make it possible to round a curve with maximum transverse acceleration.
In an other embodiment, steering angle adjustment takes place with a dependence on the operating element actuating value that is less than quadratic, and at least linearly increasing. This path of the curve of the steering angle to be adjusted as a function of the operating element actuating parameter is favorably adapted to intuitive driver behavior that conforms to control engineering.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.